igneous
Tumbled black obsidian
Volcanic glass; rhyolitic obsidian, primarily SiO₂
AI-generated identification · may be incorrectHardness about 5–5.5 Mohs; black to dark brown, vitreous luster, amorphous glassy structure, no cleavage, conchoidal fracture, specific gravity roughly 2.35–2.60. The smooth surface indicates tumbling or polishing.
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Physical properties
Hardness about 5–5.5 Mohs; black to dark brown, vitreous luster, amorphous glassy structure, no cleavage, conchoidal fracture, specific gravity roughly 2.35–2.60. The smooth surface indicates tumbling or polishing.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, usually at volcanic flows and domes during the Quaternary and older periods. The specimen appears human-processed rather than naturally fractured.
Uses & applications
Used for ornamental objects, cabochons, beads, tools, and historically razor-sharp blades; not generally used as structural stone. Jewelry-grade examples are collected for their glassy appearance.
Geological facts
Obsidian can produce exceptionally sharp edges and was used by prehistoric cultures for knives and arrowheads. Flow bands, snowflake patterns, or sheen varieties may occur, but none is clearly visible here.
Field identification & locations
Identify by its uniformly glassy black appearance, conchoidal fracture, lack of visible grains, and inability to scratch glass reliably despite its hardness. Common sources include Mexico, the western United States, Armenia, Iceland, and Japan.
